1. Field of the Invention
The present invention relates to a differential amplifier circuit and a frequency mixer for improving linearity, and more particularly to a differential amplifier circuit and a frequency mixer which achieve an improvement in linearity by offsetting, at a load side, non-linear components generated at an active element of the differential amplifier circuit, to output only linear current components.
2. Description of the Related Art
A mobile wireless transmitter/receiver or the like uses, at a front end thereof, an amplifier and a frequency mixer, which exhibit high linearity, in order to avoid a degradation in receiver sensitivity caused by an intermodulation under a limited power consumption condition.
As a method for improving the linearity of such amplifier and frequency mixer, a feedback method, a pre-distortion method, etc. have been proposed. However, such methods have a practical problem in that the application thereof to a mobile terminal is difficult due to problems such as possibility of oscillation, complexity of implementation, and noise degradation.
In order to solve the above-mentioned problem, namely, to achieve an enhancement in linearity, various differential amplifier circuits have been proposed. FIG. 1 is a circuit diagram illustrating a configuration of a conventional differential amplifier circuit. As shown in the drawing, the conventional differential amplifier circuit includes two load resistors R1 and R2, two NMOS transistors T1 and T2, and a biasing current source IBias. The two load resistors R1 and R2 have the same resistance. The two NMOS transistors T1 and T2 are set to have the same channel characteristics. Two input signals Vin+ and Vin− are applied to respective gates of the NMOS transistors T1 and T2. The differential amplifier circuit amplifies a difference between the input signals Vin+ and Vin− and outputs the resultant signal.
With regard to a differential amplifier circuit for improving linearity, Korean Unexamined Patent Publication No. 10-2006-0011742 (DIFFERENTIAL AMPLIFIER CIRCUIT AND MIXER CIRCUIT INCLUDING THE SAME) discloses a differential amplifier circuit including an auxiliary differential amplifying stage 32, as shown in FIG. 2.
In the case of the Korean unexamined patent publication, a main differential amplifying stage 21 including a pair of NMOS transistors MN1 and MN2 constituting a differential pair and an auxiliary differential amplifying stage 32 including a pair of NMOS transistors MN3 and MN4 constituting another differential pair are connected in parallel, in order to make the secondary differential coefficient of the total transconductance affecting linearity have a negative value.